/Engine/Plugins/Online/OnlineFramework/Source/Party/Public/SocialManager.h
Gives child classes a chance to append any additional data to a join request that's about to be sent to another party. This is where you'll add game-specific information that can affect whether you are eligible for the target party.
virtual void FillOutJoinRequestData ( const [FOnlinePartyId](API\Plugins\OnlineSubsystem\FOnlinePartyId) & TargetParty, [FOnlinePartyData](API\Plugins\OnlineSubsystem\Interfaces\FOnlinePartyData) & OutJoinRequestData ) const